1. Field of the Invention
The present invention relates to an image scanning apparatus in which a linear image sensor performs mechanical scanning in a direction perpendicular to the longitudinal direction of said sensor, and more particularly to such scanning apparatus adapted for use in the image signal generation in a telephoto or facsimile transmitter.
2. Description of the Prior Art
In a conventional telephoto or facsimile transmitter, a photoelectric converting element is linearly moved parallel to the axis of a rotary drum on which a photograph is wound, thereby achieving a scanning with the photoelectric converting element in combination with the drum rotation (hereinafter called horizontal scanning) and a scanning with said element in combination with the axial movement thereof (hereinafter called vertical scanning).
In such system it is difficult to exactly measure the density information of the object photograph in a short time by preliminary scanning of the entire photograph, since the horizontal scanning is conducted by mechanical rotation of the drum, which revolution cannot be made very high.
Also such rotary drum system is associated with a drawback of requiring a complicated structure for a transmissive object such as a photographic film, since an illuminating optical system and a photodetector have to be positioned across the drum made of a transparent material, namely inside and outside thereof, and have to be linearly moved in synchronism along the rotary axis thereof to achieve vertical scanning. In addition, for a small film such as of 35 mm format, there are required a high precision for the movement in the vertical scanning direction and a high resolving power in the horizontal scanning direction.
Furthermore, the image scanning apparatus for the telephoto transmitter employing a photograph wound on a rotary drum requires a long time for the preparation for transmission because of darkroom operations such as enlarging, trimming, printing and developing for printing a photograph from a photographic film, and the place of transmission is therefore inevitably limited. Besides the presence of a printing step onto the photographic paper from the film inevitably gives rise to a loss of the image information contained in the original film, such as a deterioration of the resolving power and of delicate tonal rendition.